Cultivated forms of lettuce are members of the highly polymorphic species Lactuca saliva, which is grown for its edible head and leaves. Lettuce is one of many members of the Asteraceae family. Other related plant species are, among many, Apium graveolens (celery), Foeniculum vulgare (fennel) and Cichorium intybus (chicory). As a crop, lettuce is grown commercially wherever environmental conditions permit the production of an economically viable yield.
A dicotyledonous species, L. sativa is known for the composite flowers made of individual florets. This composite flower is in fact a cluster of many small flowers, where some flowers have specialized in e.g., the “petals” that are actually ribbon-shaped flowers. Inside this area many small flowers or florets are present, each with an ovary, pappus, anthers and a style with stigmas. The florets have five fused petals to form a corolla tube. The flowers mature from the outside toward the center, with the youngest flowers in the middle.
Lettuce was first cultivated in ancient Egypt where it was used for its leaves and oil containing seeds. From there the crop spread to the Greeks and Romans. By 50 A.D., many types of lettuce were already described, and, as with many food crops, lettuce appeared in several mediaeval books, including herbal books. During the 16th through the 18th century many new varieties were developed in Europe, and cultivars from the 18th century still can be found in gardens.
Lettuce cultivars (cultivated varieties) are available in a wide range of leaf colors, leaf shapes, and textures. In addition, lettuce is rich in vitamins (A, K) and is a good source of folate and iron. In dark green lettuce types, like Romaine, higher concentrations of (3-carotene are present. The varied appearance, as well as the nutritional value of lettuce, contributes to the popularity of lettuce as a salad vegetable. Main lettuce types are: butterhead lettuce (round heads filled with thin, pliable leaves, and small, soft head with an almost oily texture) (L. sativa var. capitata), crisphead (round heads but thick, crisp leaves), which is divided further in Batavia (with open or closed, loosely filled head) (L. sativa var. capitata) and Iceberg (with overlapping leaves forming a dense firm and closed head) (L. sativa var. capitata), romaine or cos lettuce (L. sativa var. longifolia, having elongated upright leaves forming a loose, loaf-shaped head with dark green outer leaves), and leaf, cutting, or baby leaf lettuce (L. sativa var. crispa, which does not form a head). Most cultivated lettuce types are diploid; where 2n=2x=18.
Lettuce is an open pollinated crop; seeds can be grown easily since flowering plants self-pollinate because the floral structure (with a stylus growing through an anther cylinder) strongly promotes self-pollination. Lettuce is generally grown as a hardy annual; the crop is easily cultivated but requires relatively low temperatures to avoid premature flowering. Moreover, seeds from L. sativa are sensitive to high temperatures and soil salinity, both of which can affect germination.
Lettuce in general is an important and valuable vegetable crop. Therefore, it is desirable to develop new varieties of lettuce having novel and exceptional traits,